Grain　growth　in　highly　pure　nanocrystalline　Co　powders　prepared　by　high-energy　ball　milling　with　the　average　grain　size　of　about　(17±3)　nm　was　investigated　by　a　series　of　annealing　experiments.　The　characteristics　of　discontinuous　grain　growth　are　found　in　the　transition　zone　in　the　intermediate　temperature　region.　The　TEM　observations　show　that　there　are　a　number　of　low-angle　nanograin　boundaries　at　low　temperatures,　whereas　mostly　typical　high-angle　grain　boundaries　at　high　temperatures.　It　is　proposed　that,　the　rapid　grain　growth　occurring　in　the　intermediate　temperature　region　is　resulted　from　enhanced　grain　growth　promoted　by　the　stored　energy　as　an　extra　driving　force,　and　through　a　particular　dominant　mechanism　of　nanograin　rotations.